1. Field of the Invention
This invention relates to a coordinate input sheet used to specify respective coordinate input data especially when a figure is input to a computer in a so-called digitizer as an input device for written letters and figures, or when a partial area of print matter is designated and copied by a copy machine.
2. Discussion of the Prior Art
A conventional digitizer is shown in FIG. 1, which includes two coordinate input sheets 1A, 1B, each of which includes an insulating sheet and opposing electrodes 2, 3 each being disposed at a corresponding end of the insulating sheet. A surface resistor 4 is formed between the electrodes and the sheets 1A, 1B overlap so that the respective interelectrode directions are arranged at 90 degrees with respect to one another. The lower coordinate input sheet 1B is used, for example, for detecting the X-axis coordinate position of a stylus press point to be described below, while the upper coordinate input sheet 1A is used for detecting the Y-axis coordinate position.
In more detail, when any particular point on input sheet 1A is pressed in FIG. 1 and a voltage 5A is applied across the sheet 1A, a division of voltage at the point P on sheet 1A occurs and is taken as the X-axis coordinate position data from a division voltage output terminal 6 of the lower coordinate input sheet 1B. Next, by application of a voltage 5B to lower coordinate input sheet 1B, the division voltage at point P on the sheet 1B is taken as the Y-axis coordinate position data from a division voltage output terminal 7 of the upper coordinate input sheet 1A. Thus, this time-division switching operation between sheets 1A and 1B provides the X-Y coordinates of the point P where the slytus pressure is applied.
The division voltage at the input point P on the upper coordinate input sheet 1A will now be considered in greater detail. So long as the linearity of the surface resistance 4 between electrodes 2 and 3 is maintained, the division voltage at the press input point P exhibits a value inversely proportional to the distance from positive electrode 2. For example, if the inter-electrode voltage is, for example 5 V and if the point P is at the middle point between electrodes 2 and 3, a value of 2.5 V is exhibited.
However, in fact, the surface resistance 4 is affected by many factors, such as uneven printing or the like and does not exhibit a perfect resistance linearity. Thus the above 2.5 V midpoint line deviates across the coordinate input sheet as shown by a curved line in FIG. 2, from the rightfull position shown by a dot-dashed line so that even if the operator has pressed the midpoint correctly, incorrect coordinate data is often obtained. This applies likewise to lower coordinate input sheet 1B.